Balloon catheters are used for a wide variety of medical applications including angioplasty, stent deployment, embolectomy and balloon occlusion of blood vessels. A standard balloon catheter has a catheter with at least one lumen, a compliant or non-compliant balloon positioned coaxially around and bonded to the catheter at or near its distal tip. At least one of the catheter lumens, the inflation lumen, has at least one orifice positioned within the balloon lumen such that this inflation lumen is in fluid communication with the inside of the balloon. The balloon is deployed by attaching a syringe or other infusion device to the proximal end of the catheter, so that it is in fluid communication with the catheter's inflation lumen, and injecting a volume of fluid (liquid or gas) through the inflation lumen into the balloon, inflating it to a given volume or pressure. The balloon is deflated by withdrawing the fluid from the balloon lumen through the catheter's inflation lumen back into the reservoir of the syringe or other infusion device. The catheter may have additional lumens such as a guidewire lumen to facilitate maneuvering of the catheter within the body, infusion lumens to infuse fluid out the distal tip of the catheter into the patient and monitoring lumens to monitor pressure, temperature or other parameters.
There are applications where it is desirable for the fluid which inflates the balloon to flow continuously into and out of the balloon while maintaining the balloon inflated at the desired volume and pressure. One such application would be thermal ablation balloon catheters which ablate tissue using hyper or hypothermia. Balloon catheters are useful in these applications because they can be designed to conform to the tissue to be ablated once positioned in the appropriate location. Another such application would be a drug delivery balloon catheter where the balloon serves as a reservoir for a drug to be delivered through its permeable wall.
Tissue ablation is performed throughout the body. It is frequently used to destroy abnormal tissue such as malignant tumors (e.g. liver, lung) or other non-malignant tissue (e.g. endometrial, prostatic). It is also frequently used to target structurally normal tissues for a specific therapeutic effect such as cardiac tissue ablation to treat arrhythmias and more recently renal nerve ablation (“renal denervation”) to treat refractory hypertension.
Tissue ablation is most commonly performed by applying energy to the target tissue to cause irreversible cellular injury. Common energy sources for tissue ablation include radiofrequency, microwave, laser, ultrasound and cryo. Each source has its own specific characteristics, biophysical mechanism, advantages and disadvantages. All of these modalities, with the exception of cryo, ultimately act by increasing the tissue temperature to cytotoxic levels for a given period of time. Cellular injury is generally reversible below 46 C. Although there is some variability in thermal sensitivity among different tissues and cell types, irreversible cellular injury generally occurs after 60 minutes at 46 C and less than 5 minutes at 50 C.
Most clinical applications of thermal ablation have involved either large volumes of tissue (e.g. tumor ablation) or at least relatively thick tissues (e.g. cardiac ablation) where complete ablation of the target tissue is necessary for a successful therapeutic effect. Even a small volume of residual viable tissue can lead to clinical failure in the form of recurrent tumor growth, metastases from residual tumor or recurrent arrhythmias from residual pathways. For the ablation to be successful, the cells farthest from the energy source must reach the target cytotoxic temperature. The larger the distance from the energy probe to the border of the target tissue the more challenging the ablation, the more energy needs to be delivered and the higher the temperature near the probe needs to be. For example, RF ablation depends on electrical conductivity to generate heat but creating too much heat near the probe can generate charring which increases impedance and decreases the effective range of the ablation. A wide variety of technologies and techniques have been developed to accommodate the challenges of ablating across a large distances using RF (e.g. multi-electrode probes, cooling, irrigation and complex power algorithms). As a result, these tissue ablation modalities typically require a complex, external console to assure the precise amount of energy is delivered to the tissue to achieve the desired therapeutic effect. Simpler devices which use a “shotgun” approach may be ineffective or downright harmful.
The major limitation of standard balloon catheters in hyperthermic ablation applications is that the surrounding tissue serves as a powerful thermal sink. The temperature in the balloon may equilibrate with the surrounding tissue within a short period of time, shorter than the time necessary to perform the ablation, typically several minutes. For hypothermic (cryo) ablation the fluid temperature can be made so cold using liquid gases (e.g. argon, nitrogen) that the time required for the temperature to equilibrate is longer than the time it takes to ablate the tissue. For hyperthermic ablation, however, the options are more limited since the boiling temperature of most biocompatible fluids are only modestly above the temperature necessary to successfully ablate most tissues. Most tissue ablation is therefore performed using a fixed probe which is inserted into the tissue and attached to an external energy source (e.g. radiofrequency, microwave). The source continuously provides energy to the tissue as the heat dissipates into the surrounding tissue.